With the progress of small-size, lightweight and high-performance portable apparatuses, increasing demand has arisen for a secondary battery having high energy density; i.e., high capacity. In view of this tendency, a number of small-sized portable apparatuses such as cellular phones and video cameras have employed a non-aqueous lithium secondary battery such as a lithium ion battery employing a non-aqueous electrolytic solution or a lithium polymer battery, which exhibits high energy density and high voltage. Such a lithium secondary battery employs, as a negative electrode material, a carbon material such as graphite, which exhibits high charge/discharge capacity per unit mass at low electric potential nearly equal to that of lithium (Li). However, such an electrode material employed in the battery exhibits a gravimetric charge/discharge capacity nearly equal to the theoretical value, and thus the gravimetric energy density of the battery is approaching its limit. Therefore, attempts have been made to reduce the amounts of an electrode binder and an electrically conductive additive, which do not contribute to discharge capacity, so as to enhance the efficiency of the electrode.
Conventionally, fluorine-containing resins such as polyvinylidene fluoride (abbreviated as “PVDF”) and copolymers thereof have generally been employed as a binder for a negative electrode. However, recently, styrene-butadiene rubber (abbreviated as “SBR”) has been widely employed as a binder for a negative electrode, for such reasons that SBR can be added in a reduced amount to a negative electrode material, and SBR, which is used in the form of aqueous dispersion, enables to simplify an electrode production process.
Meanwhile, as an electrically conductive additive, there has been widely employed vapor grown carbon fiber, which exhibits high electrical conductivity and exerts the effect of enhancing the strength of an electrode, as compared with the case of carbon black (e.g., acetylene black), which has conventionally been employed as an electrically conductive additive. For example, Japanese Patent Laid-Open Publication (kokai) No. 4-155776 and Japanese Patent Laid-Open Publication (kokai) No. 4-237971 disclose a technique in which vapor grown carbon fiber (VGCF) is added to a graphite negative electrode, whereby the resistance of the electrode is lowered, the strength and expansion/shrinkage resistance of the electrode are enhanced, and the load characteristics and cycle life of the resultant lithium secondary battery are improved.
In the above conventional technique, vapor grown carbon fiber, which exhibits hydrophobicity, is employed in combination with PVDF as a binder, which is used in the form of organic solvent dispersion, but is not employed in combination with SBR, which is used in the form of aqueous dispersion.
A secondary battery employed in a small-sized portable apparatus is required to have a smaller size, high gravimetric energy density and high volumetric energy density. Therefore, attempts have been made to increase the amount of an electrode material charged into a battery housing by increasing the density of the electrode material, wherein charge/discharge capacity is nearly equal to the theoretical value as described above, so as to enhance the volumetric energy density of the resultant electrode and battery.
Graphite, which is at present most widely employed as a negative electrode material, has a true density of about 2.2 g/cm3, but graphite has conventionally been employed in a negative electrode having a density of about 1.5 g/cm3. When the density of the negative electrode employing graphite is increased to 1.7 g/cm3 or higher, conceivably, the volumetric energy density of the resultant battery can be enhanced. However, when the density of a negative electrode is increased, the amount of pores contained in the negative electrode is reduced, leading to problems such as deficiency of an electrolytic solution, which is generally present in the pores and plays an important role for electrode reaction, and lowering of the rate of permeation of the electrolytic solution into the negative electrode. When the amount of the electrolytic solution in the negative electrode is insufficient, the electrode reaction proceeds at a lower rate, resulting in lowering of energy density and high-speed charging/discharging performance. Meanwhile, when the electrolytic solution permeability is impaired, longer time is required for producing a battery, leading to an increase in production cost. Such problems become more pronounced in the case of a lithium polymer battery, which employs a polymer electrolytic solution of high viscosity.